1. Field of the Invention
The present invention relates to an optical cross-connect, and more particularly to an optical cross-connect that switches optical signals of N routes (N=1, 2, . . . ).
2. Description of the Related Art
Many optical network systems use the wavelength-division multiplexing (WDM) technology to interconnect network nodes such as cities and countries. WDM transmission systems involve various control techniques to, for example, deliver optical signals with different wavelengths to intended destinations, or to add or drop a desired wavelength to/from a desired route of optical signals. The latter is known as “optical add and drop multiplexing” (OADM). Also required is the feature of automatic protection switching, which enables a failed channel to be switched instantly to an alternate channel with a then-available wavelength in the event of a failure between network nodes. Optical cross-connect is a device that offers such optical switching functions, and there is an increased interest in wavelength selective switches (WSS) as a key component to realize optical cross-connects.
FIG. 33 shows the basic structure of a 1-input 3-output (1×3) WSS. The illustrated WSS 40 is formed from a spectroscopic system 41, a lens 42, and a mirror 43. The incoming light through the input port IN is reflected to one of the three output ports OUT1 to OUT3. A desired output port can be selected by varying the tilt angle of the mirror 43.
FIG. 34 shows a structure of a conventional optical cross-connect. The illustrated optical cross-connect 50 is a wavelength cross-connect (WXC) designed to switch four routes each carrying 40 WDM channels, with fixed add wavelengths. The optical cross-connect 50 includes 1×4 WSSs 51-1 to 51-4, 4×1 WSSs 52-1 to 52-4, combiners 53-1 to 53-4, splitters 54-1 to 54-4, WDM amplifiers 55-1 to 55-4 and 56-1 to 56-4, transmitters Tx, and receivers Rx.
Input routes Rin#1 to Rin#4 reach the input ports of corresponding 1×4 WSSs 51-1 to 51-4 through WDM amplifiers 55-1 to 55-4. The outputs of 4×1 WSSs 52-1 to 52-4 are directed to output routes Rout#1 to Rout#4 through WDM amplifiers 56-1 to 56-4. Signals on each add route A#1 to A#4 are entered to input ports of a corresponding combiner 53-1 to 53-4 through 40 transmitters Tx. Each drop route D#1 to D#4 has 40 receivers Rx coupled to the outputs of a corresponding splitter 54-1 to 54-4.
Input and output ports of WSSs are named as shown in the legend of FIG. 34, the order being determined by the structure of a WSS. Such ports of WSSs are connected as follows: Output port OUT1 of the first 1×4 WSS 51-1 is connected to input port IN1 of the second 4×1 WSS 52-2. Output port OUT2 of the WSS 51-1 is connected to input port IN1 of the third 4×1 WSS 52-3. Output port OUT3 of the WSS 51-1 is connected to input port IN1 of the fourth 4×1 WSS 52-4. Output port OUT4 of the WSS 51-1 is connected to input port of the first splitter 54-1.
Output port OUT1 of the second 1×4 WSS 51-2 is connected to input port IN1 of the first 4×1 WSS 52-1. Output port OUT2 of the WSS 51-2 is connected to input port IN2 of the third 4×1 WSS 52-3. Output port OUT3 of the WSS 51-2 is connected to input port IN2 of the fourth 4×1 WSS 52-4. Output port OUT4 of the WSS 51-2 is connected to input port of the second splitter 54-2.
Output port OUT1 of the third 1×4 WSS 51-3 is connected to input port IN2 of the first 4×1 WSS 52-1. Output port OUT2 of the WSS 51-3 is connected to input port IN2 of the second 4×1 WSS 52-2. Output port OUT3 of the WSS 51-3 is connected to input port IN3 of the fourth 4×1 WSS 52-4. Output port OUT4 of the WSS 51-3 is connected to input port of the third splitter 54-3.
Output port OUT1 of the fourth 1×4 WSS 51-4 is connected to input port IN3 of the first 4×1 WSS 52-1. Output port OUT2 of the WSS 51-4 is connected to input port IN3 of the second 4×1 WSS 52-2. Output port OUT3 of the WSS 51-4 is connected to input port IN3 of the third 4×1 WSS 52-3. Output port OUT4 of the WSS 51-4 is connected to input port of the fourth splitter 54-4.
The output of the first combiner 53-1 is connected to input port IN4 of the first 4×1 WSS 52-1. The output port of the second combiner 53-2 is connected to input port IN4 of the second 4×1 WSS 52-2. Likewise, the output of the combiner 53-3 is connected to input port IN4 of the third 4×1 WSS 52-3, and the output of the combiner 53-4 is connected to input port IN4 of the fourth 4×1 WSS 52-4.
This conventional optical cross-connect 50 is unable to change the wavelengths of individual add signals since it uses fixed-wavelength transmitters Tx. The optical cross-connect 50 also has to use expensive WDM amplifiers 55-1 to 55-4 and 56-1 to 56-4 each having a dispersion compensation fiber (DCF) to compensate for the large optical loss of combiners 53-1 to 53-4 and splitters 54-1 to 54-4.
FIG. 35 shows the structure of another conventional optical cross-connect, and FIG. 36 shows four types of WSSs and their input and output port numbers as a legend for WSSs shown in FIG. 35. This optical cross-connect 60 can add optical signals with desired wavelengths, unlike the optical cross-connect 50 of FIG. 34.
The optical cross-connect 60 has four 1×7 WSSs 61-1 to 61-4 on the input side to receive incoming signals from input routes Rin#1 to Rin#4. Likewise, it has four 1×4 WSSs 66-1 to 66-4 to receive add signals from add routes A#1 to A#4. On the output side, four 7×1 WSSs 62-1 to 62-4 are employed to send signals to output routes Rout#1 to Rout#4. For drop routes D#1 to D#4, four 4×1 WSSs 67-1 to 67-4 are placed.
Those WSSs are connected with each other in the following way: Output port OUT1 of the first 1×7 WSS 61-1 is connected to input port IN1 of the second 7×1 WSS 62-2. Output port OUT2 of the WSS 61-1 is connected to input port IN1 of the third 7×1 WSS 62-3. Output port OUT3 of the WSS 61-1 is connected to input port IN1 of the fourth 7×1 WSS 62-4. Output port OUT4 of the WSS 61-1 is connected to input port IN1 of the first 4×1 WSS 67-1. Output port OUT5 of the WSS 61-1 is connected to input port IN1 of the second 4×1 WSS 67-2. Output port OUT6 of the WSS 61-1 is connected to input port IN1 of the third 4×1 WSS 67-3. Output port OUT7 of the WSS 61-1 is connected to input port IN1 of the fourth 4×1 WSS 67-4.
Output port OUT1 of the second 1×7 WSS 61-2 is connected to input port IN1 of the first 7×1 WSS 62-1. Output port OUT2 of the WSS 61-2 is connected to input port IN2 of the third 7×1 WSS 62-3. Output port OUT3 of the WSS 61-2 is connected to input port IN2 of the fourth 7×1 WSS 62-4. Output port OUT4 of the WSS 61-2 is connected to input port IN2 of the first 4×1 WSS 67-1. Output port OUT5 of the WSS 61-2 is connected to input port IN2 of the second 4×1 WSS 67-2. Output port OUT6 of the WSS 61-2 is connected to input port IN2 of the third 4×1 WSS 67-3. Output port OUT7 of the WSS 61-2 is connected to input port IN2 of the fourth 4×1 WSS 67-4.
Output port OUT1 of the third 1×7 WSS 61-3 is connected to input port IN2 of the first 7×1 WSS 62-1. Output port OUT2 of the WSS 61-3 is connected to input port IN2 of the third 7×1 WSS 62-3. Output port OUT3 of the WSS 61-3 is connected to input port IN3 of the fourth 7×1 WSS 62-4. Output port OUT4 of the WSS 61-3 is connected to input port IN3 of the first 4×1 WSS 67-1. Output port OUT5 of the WSS 61-3 is connected to input port IN3 of the second 4×1 WSS 67-2. Output port OUT6 of the WSS 61-3 is connected to input port IN3 of the third 4×1 WSS 67-3. Output port OUT7 of the WSS 61-3 is connected to input port IN3 of the fourth 4×1 WSS 67-4.
Output port OUT1 of the fourth 1×7 WSS 61-4 is connected to input port IN3 of the first 7×1 WSS 62-1. Output port OUT2 of the WSS 61-4 is connected to input port IN3 of the second 7×1 WSS 62-2. Output port OUT3 of the WSS 61-4 is connected to input port IN3 of the third 7×1 WSS 62-3. Output port OUT4 of the WSS 61-4 is connected to input port IN4 of the first 4×1 WSS 67-1. Output port OUT5 of the WSS 61-4 is connected to input port IN4 of the second 4×1 WSS 67-2. Output port OUT6 of the WSS 61-4 is connected to input port IN4 of the third 4×1 WSS 67-3. Output port OUT7 of the WSS 61-4 is connected to input port IN4 of the fourth 4×1 WSS 67-4.
Output port OUT1 of the first 1×4 WSS 66-1 is connected to input port IN4 of the first 7×1 WSS 62-1. Output port OUT2 of the WSS 66-1 is connected to input port IN4 of the second 7×1 WSS 62-2. Output port OUT3 of the WSS 66-1 is connected to input port IN4 of the third 7×1 WSS 62-3. Output port OUT4 of the WSS 66-1 is connected to input port IN4 of the fourth 7×1 WSS 62-4.
Output port OUT1 of the second 1×4 WSS 66-2 is connected to input port IN5 of the first 7×1 WSS 62-1. Output port OUT2 of the WSS 66-2 is connected to input port IN5 of the second 7×1 WSS 62-2. Output port OUT3 of the WSS 66-2 is connected to input port IN5 of the third 7×1 WSS 62-3. Output port OUT4 of the WSS 66-2 is connected to input port IN5 of the fourth 7×1 WSS 62-4.
Output port OUT1 of the third 1×4 WSS 66-3 is connected to input port IN6 of the first 7×1 WSS 62-1. Output port OUT2 of the WSS 66-3 is connected to input port IN6 of the second 7×1 WSS 62-2. Output port OUT3 of the WSS 66-3 is connected to input port IN6 of the third 7×1 WSS 62-3. Output port OUT4 of the WSS 66-3 is connected to input port IN6 of the fourth 7×1 WSS 62-4.
Output port OUT1 of the fourth 1×4 WSS 66-4 is connected to input port IN7 of the first 7×1 WSS 62-1. Output port OUT2 of the WSS 66-4 is connected to input port IN7 of the second 7×1 WSS 62-2. Output port OUT3 of the WSS 66-4 is connected to input port IN7 of the third 7×1 WSS 62-3. Output port OUT4 of the WSS 66-4 is connected to input port IN7 of the fourth 7×1 WSS 62-4.
For the first add route A#1, the optical cross-connect 60 has forty wavelength-tunable transmitters Txv-1 to Txv-40, five 8×1 WSSs 63a-1 to 63a-5, a 5×1 WSS 64-1, and a WDM amplifier 65-1. The forty transmitters Txv-1 to Txv-40 transmit forty optical signals, respectively. Each WSS 63a-1 to 63a-5 receives eight out of forty optical signals from the transmitters Txv-1 to Txv-40 and selectively outputs one optical signal to the 5×1 WSS 64-1. The 5×1 WSS 64-1 then chooses one optical signal from among five optical signals received from the five 8×1 WSSs 63a-1 to 63a-5. The WDM amplifier 65-1 amplifies the output of this WSS 64-1 before feeding it as an add signal to the first 1×4 WSS 66-1. Other three add routes A#2 to A#4 have the same hardware structure as the first add route A#1 described above.
For the first drop route D#1, the optical cross-connect 60 has a WDM amplifier 68-1, a 1×5 WSS 69-1, and five 1×8 WSS 70a-1 to 70a-5, and forty receivers Rx-1 to Rx-40. The WDM amplifier 68-1 amplifies a drop signal produced by the first 4×1 WSS 67-1. Five outputs of the WSS 69-1 are connected to five input ports of different 1×8 WSSs 70a-1 to 70a-5 to route the drop signal to any one of their forty output ports as specified. Eight outputs of the first 1×8 WSS 70a-1 are directed to a group of receivers Rx-1 to Rx-8, as are the outputs of the second 1×8 WSS 70a-2 to a subsequent group of receivers Rx-9 to Rx-16. Likewise, other three 1×8 WSSs 70a-3 to 70a-5 provide their outputs to corresponding receiver groups Rx-17 to Rx-24, Rx-25 to Rx-32, and Rx-33 to Rx-40. Note that only one of eight signals of each group carries active drop signal information. Each receiver Rx-1 to Rx-40 receives an optical drop signal (if present). Other three drop routes D#2 to D#4 have the same hardware structure as the first drop route D#1.
The number of WSS ports on the input and output routes may be increased or decreased depending on how many routes are to be switched. In the case where the network system needs more wavelengths in WDM transmission, the WSSs on add and drop routes have to be replaced to provide more ports.
The above conventional optical cross-connect 60 realizes the selective add-wavelength WXC functions by combining 1×7/7×1 WSSs, 1×4/4×1 WSSs, 1×8/8×1 WSSs, and 1×5/5×1 WSSs, but without using optical combiners or splitters. Like the one shown in FIG. 34, the optical cross-connect 60 of FIG. 35 can switch cross-connections between four routes each having 40 WDM channels. For example, the WSSs 63a-1 to 63a-5 and 64-1 on the first add route A#1 perform fast switching to add 40-ch WDM signals to a desired output route through the WDM amplifier 65-1 and 1×4 WSS 66-1.
As an example of such conventional optical cross-connecting techniques, Japanese Patent Application Publication No. 2004-343231, paragraphs 0017 to 0019, FIG. 1, proposes an optical cross-connect having input and output ports for individual channels to enable interface with existing network facilities.
The above-described architectures of conventional optical cross-connects may not be optimal in terms of how they use WSSs. Specifically, the optical cross-connect 60 of FIG. 35 includes many WSSs in its add/drop sections. This design is advantageous over the optical cross-connect 50 of FIG. 34 in its smaller signal loss since no optical combiners or splitters are required. It also eliminates the need for costly WDM amplifiers with DCF. On the other hand, the increased use of WSSs also means increased product cost, as well as an increased number of interconnections between them, thus making a cross-connect device more complex and larger.
The conventional optical cross-connect 60 provides various switching patterns for optical cross-connection, and many of them can work simultaneously. In reality, however, the simultaneity of cross-connections is not always required in real-world applications. FIGS. 37 and 38 show tables T61 and T62 representing the simultaneous feasibility of optical cross-connections that the optical cross-connect 60 of FIG. 35 can or cannot provide. Specifically, table T61 summarizes simultaneous feasibility of optical cross-connections between input route Rin#1 and add route A#1. Table T62, on the other hand, summarizes the same for cross-connections between input route Rin#1 and drop route D#1.
Table entries having a value of “YES” mean that the corresponding switching paths can work simultaneously. The top-left entry of table T61, for example, is marked “YES,” indicating that an add switching path from add route A#1 to output route Rout#1 can work together with a through switching path from input route Rin#1 to output route Rout#2.
Table entries having a value of “NO” indicate that the corresponding combination of switching paths cannot work simultaneously. Reversely stated, one of those paths can work selectively. See the entry at column 2, row 1 of table T61, for example. This entry is marked “NO,” meaning that an add switching path from add route A#1 to output route Rout#2 cannot work together with a through switching path from input route Rin#1 to output route Rout#2. This is because these two paths collide at an output port of the second 7×1 WSS 62-2. The other table T62 is supposed to be interpreted in the same way.
FIG. 39 shows yet another table representing simultaneous feasibility of optical cross-connections. This table T63 is a logical product of table T61 and table T62, which is produced by performing a logical operation on each corresponding pair of entries of those two tables. Specifically, let x and y represent respectively the values of table T61 and table T62 at a particular column and row. The logical product of two table entries (x, y) takes a value of “YES” if (x,y)=(YES,YES), or a value of “NO” if (x,y)=(YES,NO) or (NO,YES) or (NO,NO). The resulting value is then set to a corresponding entry of table T63.
Referring to table T63, the entries having a value of “YES” indicate that the corresponding through switching path and add/drop switching paths can work simultaneously. See the entry at column 3, row 1, for example. This entry has a value of “YES,” meaning that a through switching path from input route Rin#1 to output route Rout#2, an add switching path from add route A#1 to output route Rout#3, and a drop switching path from input route Rin#3 to drop route D#1 can all work at the same time.
Other entries of table T63 have a value of “NO” to indicate that either the corresponding through switching path or the corresponding add/drop switching paths can work selectively. See the top-left entry of table T63, for example. This table entry indicates that the optical cross-connect 60 does not allow simultaneous use of a through switching path from input route Rin#1 to output route Rout#2, an add switching path from add route A#1 to output route Rout#1, and a drop switching path from input route Rin#1 to drop route D#1. That table entry, however, implies that either the through switching path alone or the add/drop switching paths can be implemented.
In reality, however, it may not be necessary to make available every combination of a through path and add/drop paths for simultaneous use. Rather, most cases require either the former or the latter alone. Table T63, however, shows many switching patterns marked “YES.” This means that the conventional optical cross-connect 60 is not optimized as to the simultaneity of its switching paths. The entire circuit of an optical cross-connect will be smaller in size if it is possible to reduce some simultaneous switching patterns of through and add/drop paths (which means reducing “YES” and increasing “NO” in table T63).